Survey-mode measurementsand analysis

M. Floyd K. PalamartchoukMassachusetts Institute of Technology Newcastle University

GAMIT-GLOBK courseUniversity of Bristol, UK

12–16 January 2015

Material from R. King, T. Herring, M. Floyd (MIT) and S. McClusky (now ANU)

Measurement Strategies I: Occupation Time

Given time and personnel constraints, what are the trade-offs between between spatial and temporal density?

Ideally, you would like for the white-noise position uncertainty for an occupation to contribute to the velocity uncertainty at a level less than the usually dominant long-period correlated noise.

Typical white-noise uncertainties (Horizotnal and Vertical) as a function of occupation time:

6-8 hrs: 2-2.5 mm H, 5-10 mm V

12-24 hrs: 1.0-1.5 mm H, 3-5 mm V

36-48-hrs: 0.7-1.0 mm H, 2-4 mm V

Observations over 3 or more days will give you more redundancy

Observations of 5 or more days will be necessary for mm-level vertical uncertainties

If your region has few continuous stations, you should consider running one or two survey-mode stations for the entire time of the survey to provide continuity

Precision v session length for network processing

Measurement Strategies II: Monuments and Instrumentation

Issues in site and antenna selection :

Monument stability

Accessibility

Ease of setup

Multipath

Vandalism

• There is no clear prescription for all cases; let’s look at some examples

Three primary mounting options

Tripod with optical or physical plummet

Spike mount

Mast Courtesy UNAVO web page

Low mount in a good enviroment

STVPSteven’s Pass, Cascades Range in western Washington

18-cm spike mount

No long-term repeatability yet, but 44 hrsof observations in 2012 give formaluncertainties 0.5 mm horizontal, 3 mm vertical. Note minimal long-period signal Scattering.

Low mount in a dirty environment

B059Roadside meadow in western Washington 12.5-cm spike mount

Two 24-hr measurements in 2012 agree at 1 mm horizontal, 4 mm though formal uncertainties 2 mm, 10mm due to high random noise (diffuse multipath or water vapor?) Note minimal long-period signal scattering. Long-term scatter 3 mm horizontal, 5 mm vertical (monument instablity?)

High mount in a dirty environment

C033Old survey mark in dirt in central Washington.Tripod mount. (Train blockage was short-lived)

2012 19-hr session and 5-hr session agree at 1.5 mm horizontal, 3 mm vertiical. Long-term repeatability 2 mm horizontal, 12 mm vertical.

Surprisingly little short-period multipath (dry dirt?)

Low mount on a slope

LYFRRocky river bank in eastern Oregon 12.5-cm spike mount

Single 14-hr session . Long-period multipath due to slope and/or reflective rocks ?

Special Characteristics of Survey-mode Data

• Editing is critical: every point counts• Usually combined with cGPS data to provide continuity and a tie to the

ITRF• Appropriate relative weighting needed in combining with cGPS data• Antenna meta-data may be more complicated• Heights may be problematic if different antennas used• Seasonal errors behave differently than in cGPS data: best strategy is to

observe at the same time of the year (unlike cGPS, which has minimal seasonal sensitivity at 1.5, 2.5, 3.5 ….years total span)

Analysis Strategy • Generate time series and aggregated h-files for each survey

– Use spans less than ~ 30 days to avoid biasing the position estimate from an in correct velocity

– Include cGPS data only on days when sGPS data are available to maintain common-mode cancellation

– Strenghens the sGPS positions estimates within each survey and allows better assessment of the long-term statistics

– Edit carefully the daily values within each span• Generate time series and a velocity using the aggregated h-files from a span of 3 or

more years– Edit carefully the long-term time series– Add 0.5 of white noise (sig_neu) to the cGPS estimates from each span to avoid

overweighting the cGPS position estimates– Use a separate (ie.g PBO) analysis of the daily cGPS time series to get the

appropriate RW (mar_neu) values for each cGPS site; use the median RW for the sGPS sites.

• See sGPS_recipe.txt for detailed commands

SARG

DALL

BURN

217U

Locations of one continuous (BURN) and 3 survey-mode sites for time series shown in next slides

Time series of monthly position estimates for continuous site BURN

Wrms ~ 1 mm N,E ~ 3 mm U

Rate uncertainties < 0.2 mm/yr N,E 0.7 mm/yr U do not include random walk added for velocity estimates

SARG

DALL

BURN

217U

Next slide shows time series for survey-mode site 217U

Note consistency with nearby sites

Time series for survey-mode site 217U

Position estimates based on 8-24 hr occupations

Note < 1 mm rate uncertainties due to 7-yr time span

SARG

DALL

BURN

217U

Next slide shows time series for survey-mode site SARG

Horizontal time series for survey-mode site SARG

Position estimates based on 8-24 hr occupations

Note 1 mm wrms and

< 1 mm rate sigmas

SARG

DALL

BURN

217U

Next slide shows time series for survey-mode site DALL

Note consistency with nearby sites except continuous site GWEN

Position estimates based on 8-24 hr occupations

Rate sigmas < 1 mm/yr and consistent with surrounding sites even with velocities determined essentially by two occupations 3 yrs apart

Horizontal time series for survey-mode site DALL